The invention generally relates to a hydraulic valve, more specifically to a positive-seating cartridge check valve that can be hydraulically unlocked.
Such a cartridge check valve is a positive-seating two 2/2-way valve with a pilot operated non-return function. It blocks flow with minimum leakage and allows free flow when hydraulically unlocked. It is for example used to hermetically shut-off a working port of a hydraulic cylinder, in order to prevent creep movements of the piston when the latter is subjected to an external load. For implementing the shut-off function, such a valve shall use the pressure to be shut-off and not rely on an additional external pressure source.
Cartridge type 2/2-way valves have been widely used in hydraulic control systems as flow, directional and pressure-control valves for about 30 years. For a more detailed description of cartridge check valves it is for example referred to the article "Cartridge check valves: new option for hydraulic control" by David C. Downs, in "Machine Design", Vol. 52, No. 28 dated Dec. 11, 1980, Cleveland USA, pp. 143-147. A cartridge seat-type 2/2-way valve is likewise described in patent specification DE-A-36 19 927.
A standard cartridge seat-type 2/2-way valve comprises:
1) a valve sleeve for mounting in a control block with a front main flow connection A, a lateral main flow connection B and a valve seat between the main flow connections A and B; PA0 2) a valve piston or poppet, which is fitted so as to be axially movable in a guide hole in the valve sleeve, its first end forming a closing cone for the valve seat; PA0 3) a closing spring, which is assigned to the valve piston in such a way that it exerts a closing force on the latter in the direction of the valve seat, and PA0 4) a valve cover, which closes the valve sleeve axially and delimits therein a control chamber, in which the other end of the valve piston forms a front control surface A.sub.X.
FIG. 1 in the enclosed drawings shows a standard 2/2-way cartridge valve used as pilot operated non-return valve in a hydraulic circuit. To block the closing cone by the pressure p.sub.A, i.e. the pressure to be shut off in connection A, the pressure p.sub.A is admitted to the control chamber via a controllable seat-type shuttle valve integrated in the valve cover. The valve piston is under equilibrium of forces, if the following equation is satisfied: EQU p.sub.A A.sub.A +p.sub.B A.sub.B =p.sub.A A.sub.X +F.sub.spring
as A.sub.X .apprxeq.A.sub.A +A.sub.B, the opening pressure p.sub.B is: EQU p.sub.B p.sub.A +F.sub.spring /A.sub.B
To open the valve, the control pressure connection Y is pressurized, whereas the control pressure connection X is discharged to the tank. The shuttle valve in the valve cover now connects the unpressurized control chamber via the control pressure connection X to the tank. The pressure forces acting on the seat surface A.sub.A and the annular surface A.sub.B of the valve piston, push the latter against the spring force towards a stop on the valve cover and thus open the valve. An electromagnetically controlled seat-type 3/2-way valve can be used instead of the hydraulically controlled seat-type shuttle valve. In the shuttle valve, as well as in the 3/2-way valve, it must be ensured that no leakage occurs between control pressure connection C and Z2 on the one hand and the control pressure connection X on the other hand. Otherwise, satisfactory maintenance of pressure in connection A cannot be ensured. In the hydraulic circuit shown in FIG. 1, it is also disadvantageous that the closing against the pressure p.sub.A, which is initiated by pressure relief of Y, is effected essentially by the closing spring. This leads indeed to undesirably long closing times.
An important improvement with regard to the closing time is achieved by the use of so-called actively controlled 2/2-way cartridge valves according to FIG. 2. The essential difference between the actively controlled 2/2-way cartridge valve according to FIG. 2 and a standard 2/2-way cartridge valve according to FIG. 1 is that the valve sleeve has a stepped guide hole for a valve piston with a shoulder with enlarged cross-section A.sub.X. This shoulder of the valve piston delimits inside the guide hole an annular second control chamber, which is fluidically coupled to the control pressure connection Y. In this second control chamber the shoulder of the valve piston forms an additional annular control surface A.sub.Y acting in the opening direction. To block the closing cone by the pressure p.sub.A, i.e. by the pressure to be shut off in connection A, the front control chamber must be exposed to a control pressure p.sub.X =p.sub.A via a controllable seat-type shuttle valve, which is e.g. integrated in the valve cover, and the second control chamber must be discharged into the tank. Accordingly an additional hydrostatic force p.sub.A (A.sub.X -A.sub.C) supports the spring force during the closing movement. This active hydrostatic closing force enables the closing time to be reduced by a factor of 10 or more compared to the solution in FIG. 1. Because of the enlarged control surface A.sub.X and the resulting larger control oil volume, correspondingly large throughflows in the pilot multi-way valve are required to achieve short switching times. As in the solution according to FIG. 1 it must likewise be ensured in the solution according to FIG. 2 that no leakage occurs between control pressure connection C and Z2 on the one hand and control pressure connection X on the other hand.
With a pressureless second control chamber the valve piston closing the valve seat is under equilibrium of forces, if the following equation is satisfied: EQU p.sub.A A.sub.A +p.sub.B A.sub.B =p.sub.A A.sub.X +F.sub.spring
The opening pressure p.sub.B is: EQU p.sub.B =p.sub.A (A.sub.X -A.sub.A)/A.sub.B +F.sub.spring /A.sub.B
It will be appreciated that this pressure p.sub.B is substantially higher than in the valve in FIG. 1.
By application of pressure to the control surface A.sub.Y and pressure relief of the control surface A.sub.X the valve piston can be displaced actively towards the upper end stop.
A 2/2-way cartridge valve used as pilot-operated control valve is known from EP-A-0634577. The essential difference between this valve and the 2/2-way cartridge valve according to FIG. 2 is that a connecting duct through the valve sleeve links the annular second control chamber to the connection B. To shut off a pressure p.sub.A in connection A, the first control chamber is exposed to a control pressure p.sub.X =p.sub.A. The cross-section of the piston shoulder in the first control chamber, which is enlarged in relation to the cross-section of the valve seat, generates a hydrostatic closing force, which augments the closing force of the closing spring, so that the connection A is reliably shut off against the pressure p.sub.A. As in the standard 2/2-way cartridge valve, essentially the closing spring is effective in the closing against the pressure p.sub.A in the connections A and B, which leads to undesirably long closing times. If, by contrast, the first control chamber is discharged into the tank, the preceding 2/2-way control valve opens at a corresponding pressure in the main flow connection A and/or B against the closing force of the closing spring and accordingly does not have a real shut-off direction.
A variant of the 2/2-way valve described above, which can be used as a non-return valve, either with blocking direction from connection B to connection A or with blocking direction from connection A to connection B, is likewise described in EP-A-0634577. If the 2/2-way valve described above is to be used as non-return valve with blocking direction from connection B to connection A, the first and second control chambers should be connected to each other via a hole through the piston shoulder. By contrast, a blocking direction form connection A to connection B is achieved, if the first control chamber is connected to the connection A via a hole in the valve piston. However, these non-return valves cannot be opened hydraulically.